Laser diodes used in consumer devices today are supposed to be limited to produce light within prescribed safety levels. Because laser light is coherent and highly directional, laser light can easily injure one's eye even at relatively low light levels. Despite precautions that are taken to maintain laser light within safe operating levels, there is a concern that light from a laser diode may still present a safety hazard in certain situations, such as where a fault has occurred in the circuit driving the laser diode. Because a laser diode emits light in an amount that depends upon the amount of electric current passing through the laser diode (also referred to as the amount of current driving the laser diode), current driver circuits have been developed for regulating the electric current passing through laser diodes. Such current driver circuits should allow the laser diode to operate with sufficient optical power over a defined environmental envelope of operation throughout the life of the product. This balance between performance and safety can result in the need for tight operating tolerances for the current driver circuit. For example, a current driver circuit may be required to produce optical power within a range of plus-or-minus five percent of a desired optical power.
In addition, a current driver is often designed to maintain a proper current even where a component in the current driver fails. Typically, a current driver circuits that allow for a relatively wide operating tolerance are protected with limiting circuits that have limiting resistors. However, such limiting circuits are not always appropriate. For example, where a current driver circuit can operate over a wide range of power supply voltages or forward voltages across the laser diode, such limiting circuits can allow undesirably large increases in current to occur.